The present invention relates to power supplies, and more particularly to AC/DC adapter circuits in power supplies.
The voltage of the alternating current (AC) sources from typical outlets vary from country to country, mostly between 90V and 265V. The alternating current/direct current (AC/DC) adapters for laptop computer power supplies and for batteries of most mobile products need to be compatible for these various AC sources.
FIG. 1 is a circuit diagram illustrating a conventional AC/DC adapter. This circuit 100 is compatible with AC sources ranging from 90V to 265V. The circuit 100 comprises a rectifier bridge 104 which fully rectifies the AC voltage input from the AC source 102. The AC voltage from the rectifier bridge 104 charges the input capacitor C1106, developing a DC voltage. Thus, the DDC voltage across C1106 can vary from a low 100V DC to a high 375V DC. The transformer T9108 is a flyback transformer that stores the energy when transistor Q2110 is xe2x80x9con xe2x80x9d, and releases the energy to the output when Q2110 is xe2x80x9coffxe2x80x9d. The DC output voltage is filtered by diode D8112 and capacitor C5114. The DC output voltage is regulated by an error amplifier 116, which changes the pulse width of the switcher using the pulse width modulator 118 (PWM).
However, because T9108 is a storage transformer, its size is large. Since Q2110 is a high current, high voltage transistor required for wide range of input voltage, its size is large. In addition, due to the very wide range of AC input voltage, the circuit 100 cannot operate at a very high frequency, i.e., greater than 150 KHz. For these reasons, the size of the circuit 100 is very large. Due to its large size and its requirement to accommodate a wide range of AC input voltages, the circuit 100 is inefficient.
Accordingly, there exists a need for an adapter circuit which improves efficiency and size. The present invention addresses such a need.
The present invention provides an adapter circuit which improves efficiency and size. The adapter circuit includes: a selective voltage doubler which approximately doubles an input voltage with a low line connector and does not with a high line connector; a switcher coupled to the selective voltage doubler; a resonant circuit coupled to the switcher; a first diode and a second diode coupled to the resonant circuit; and a filter coupled to the first and second diodes. When a low line connector is used, the input voltage is doubled and a one diode drop is used. When a high line connector is used, the input voltage is not doubled and a two diode drop is used. This allows the adapter circuit to be significantly reduced in size and weight. It also has low power loss.